This disclosure relates generally to orifice fittings for measuring fluid flow rates through pipes or other conduits. More particularly, the disclosure relates to a drainage system in a horizontally-installed orifice fitting for measuring fluid flow.
In pipeline operations and other industrial applications, flow meters are used to measure the volumetric flow rate of a liquid or gaseous flow stream moving through a piping section. Flow meters are available in many different forms. One common flow meter is an orifice meter, which includes an orifice fitting connected to the piping section. The orifice fitting serves to orient and support an orifice plate that extends across the piping section perpendicular to the direction of the flow stream. The orifice plate is generally a thin plate that includes a circular opening, or orifice, which is typically positioned concentric with the flow stream.
In operation, when the flow stream moving through the piping section reaches the orifice plate, the flow is forced through the orifice, thereby constricting the cross-sectional flow area of the flow. Due to the principles of continuity and conservation of energy, the velocity of the flow increases as the stream moves through the orifice. This velocity increase creates a pressure differential across the orifice plate, which can be measured and used to calculate the volumetric flow rate of the flow stream moving through the piping section.
A dual chamber orifice fitting embodies a special design that enables the orifice plate to be removed from the fitting without interrupting the flow stream moving through the piping section. A common dual chamber orifice fitting 10 is illustrated in cross-section by FIG. 1. Orifice fitting 10 includes a body 12 and a top 14 coupled thereto. Body 12 encloses a lower chamber 16 in fluid communication with a bore 18 extending through body 12. When orifice fitting 10 is coupled to a piping section, bore 18 is in fluid communication with a flowbore through the piping section. Top 14 encloses an upper chamber 20 and is connected to body 12 by one or more fastening devices 22. Aperture 24 defines an opening connecting upper chamber 20 to lower chamber 16. A valve seat 26 is connected to top 14 about aperture 24. A valve plate assembly 28 sealingly engages valve seat 26. Valve plate assembly 28 is slidably actuated by a gear shaft 30 to open and close aperture 24. Orifice fitting 10 further includes lower drive 32 and an upper drive 34 which are operable to move an orifice plate carrier 36 vertically between lower chamber 16 and upper chamber 20 when aperture 24 is open. Orifice plate carrier 36 sealing engages the wall of body 12 when disposed within bore 18 and supports an orifice plate 38 having an orifice 40 extending therethrough.
In operation, orifice plate carrier 36 is disposed within lower chamber 16, and aperture 24 is closed by valve plate assembly 28, thereby hydraulically isolating lower chamber 16 from upper chamber 20. Pressurized fluid flow in bore 18 passes through orifice 40 of orifice plate 38. Pressure up and downstream of orifice plate 38 is measured via instrumentation installed within meter tap holes 42. The measured pressure differential across orifice plate 38 is then used to estimate the rate of fluid flow through fitting 10. In order to obtain accurate estimates of the flow rate through fitting 10, all of the flow moving through the piping section must pass through orifice 40. If any flow bypasses, or flows around, orifice 40, an error in the measurement of the pressure differential across orifice plate 38 occurs. To prevent flow from bypassing orifice 40, a seal 44 is disposed between orifice plate 38 and orifice plate carrier 36.
When the pressure within lower chamber 16 is lower than that of bore 18, the pressure within bore 18 will tend to urge orifice plate carrier 36 upward and into lower chamber 16, potentially causing misalignment between orifice 40 and bore 18 that can decrease measurement accuracy. Further, seal 44, which is usually constructed from an elastomer or polymer, may fail due to the pressure differential between bore 18 and lower chamber 16. In order to counter the pressure differential, an equalization flow path or weephole 46 is included between lower chamber 16 and bore 18. Weephole 46 enables fluid communication between bore 18 and lower chamber 16, and thus, pressure equalization across orifice plate carrier 36. Weephole 46 is preferably positioned upstream of orifice 40 so as to be located in the region of highest pressure within bore 18.
Gas flow passing through orifice fittings, such as fitting 10, may contain moisture. Over time, some of that moisture may collect within the fitting. Due to the effect of gravity, the moisture will collect at the lowest point of the fitting body. The lowest point of body depends on the installed orientation of the fitting. For example, orifice fitting 10 is designed to be installed in the vertical orientation, as shown in FIG. 1. In that orientation, moisture accumulates in a region 48 proximate the base of bore 18, particularly near the upstream side of plate 38. Given sufficient time, moisture accumulation may rise to a level at or above meter tap holes 42, thereby exposing flow measuring components disposed within meter tap holes 42 to the accumulated moisture. Exposure of these components to moisture may cause failure of these components, impeding the ability of orifice fitting 10 to provide accurate flow measurements.
Turning to FIG. 2, body 12 of orifice fitting 10 may be modified to include one or more drain ports 50 to alleviate the moisture accumulation in region 48. These ports 50 may be capped with plugs 52 that are removable to allow drainage of moisture from body 12. The effectiveness of ports 50 in enabling drainage of moisture from body 12 is dependent upon the installed orientation of orifice fitting 10. When fitting 10 is installed in the vertical orientation, such that a side 60 of body 12 is proximate the ground 70 and top 14 of fitting 10 is distal the ground 70, as shown in FIG. 2, drain ports 50 are positioned proximate the lowest portion of body 12. Consequently, moisture within fitting 10 tends to accumulate proximate drain ports 50 and may be drained from fitting 10 when plugs 52 are removed from ports 50.
However, orifice fitting 10 is frequently installed in a horizontal orientation, as illustrated in FIG. 3. In the horizontal orientation, one set of meter tap holes 42 is located in a lower side 54 of body 12 proximate the ground 70, while the other set of meter tap holes 42 is located in an upper side 56 of body 12 distal ground 70. Because moisture within body 12 will tend to accumulate near lower side 54, flow measuring components installed within meter tap holes 42 proximate this side 54 are vulnerable to the accumulated moisture. Hence, these holes 42 are not typically fitted with flow measuring components. Instead, the flow measuring components will be coupled to meter tap holes 42 in upper side 56 of body 12, away from the region where moisture will collect.
During operation, moisture, including particulates such as sand, will collect over lower side 54 of body 12, and in particular within a region 58 proximate lower drive 32. Region 58 is lower than the drain port 50 formed in lower side 54 of body 12. Consequently, when plug 52 is removed from this drain port 50 to drain accumulated moisture from body 12, moisture and particulates within region 58 will not be removed from body 12. Overtime, this moisture and sediment may cause damage to fitting 10 at this location.
Further, when plate carrier 36 is moved between lower chamber 16 and upper chamber 20, valve plate assembly 28 slides relative to valve seat 26 to open and close aperture 24. Due to the presence of moisture accumulation in body 12, particulates in body 12 may get agitated with movement of plate carrier 36 between chambers 16, 20 and adhere to the interfaces between valve plate assembly 28 and valve seat 26. These particulates may prevent effective sealing between valve plate assembly 28 and valve seat 26.
Accordingly, there remains a need in the art for a drainage system for a horizontally installed orifice fitting that addresses these and certain other limitations of the prior art.